Femtosecond high-contrast pulses from a parametric generator

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Femtosecond high-contrast pulses from a parametric generator pumped by the self-compressed second harmonic of a Nd:glass laser R. Danielius, A. Dubietis, G. Valiulis, and A. Piskarskas Laser Research Center, Vilnius University, Sauletekio ˙ Avenue 10, 2054 Vilnius, Lithuania Received May 18, 1995 Nearly transform-limited femtosecond pulses tunable from 635 to 3000 nm were generated by a traveling-wave type II phase-matching barium borate parametric generator pumped by the self-compressed second harmonic of a Nd:glass laser. A total conversion eff iciency of 10% with output pulses as short as 200 fs has been achieved. Numerical simulation indicated that the relative amount of energy contained in the output pulse wings was reduced by ,4 3 104 compared with that in the pump.  1995 Optical Society of America

Second-harmonic (SH) pulse shortening is observed in type II phase-matching crystals with appropriate group delay relations between two orthogonally polarized pump pulses and a SH pulse.1 – 3 So far, the best conditions for SH pulse compression have been found in a potassium dihydrogen phosphate (KDP) crystal with picosecond Nd:glass 1055-nm pulses at input.4,5 A peak power conversion rate as high as 240% and fivefold pulse shortening have been reported.2 However, the self-compressed pulses exhibit a pedestal that contains a signif icant amount of the pulse energy and has a width comparable with that of the input pulse. The contrast may be improved by introduction of a parametric amplifier stage.1,6 In particular, for selfcompressed SH pulses it has been achieved only at rather modest conversion rates, because the parametric amplif ier was seeded by a long pulse1 and wings became pronounced as soon as the parametric amplifier was driven into saturation. In this Letter we report what is to our knowledge the first application of self-compressed SH pulses of a picosecond Nd:glass laser to the generation of femtosecond tunable high-contrast pulses by a traveling-wave three-stage optical parametric generator (TOPG). The tuning range of 635–3000 nm was covered by pulses as short as ,200 fs with a time–bandwidth product of ,0.6. The pulse shape was measured to be nearly Gaussian over the dynamic range of the autocorrelator (103 ). Numerical simulation indicated that the amount of energy contained in the TOPG output pulse wings was reduced by a factor of ,4 3 104 compared with that in the pump pulse satellites. As the laser source we used a fiberless chirpedpulse-amplif ication-based Nd:glass system consisting of a feedback-controlled f lash-lamp-pumped master oscillator, a regenerative amplif ier, a stretcher, and compressor stages. It delivered , 4-mJ, 1.2 –1.3-ps pulses at 1055 nm with a 2-Hz repetition rate. The SH pulse compressor was made of two 2-cm-long KDP crystals cut for type II SH phase matching. The first crystal was used to introduce an appropriate delay between the o and e components of the pump pulse; the second one was a frequency doubler. The TOPG that we used (Topas 501, Light Conversion Ltd.) was a triple-pass device with a pump independent of 0146-9592/95/212225-03$6.00/0

all three stages. The tunability range of the signal wave was 635–1055 nm; it was 1055–3000 nm for the idler wave. The nonlinear crystal employed was 4-mm-long type II phase-matching b-barium borate (BBO) cut at u ­ 30±. Pulse duration measurements of the SH and TOPG output were performed with a home-built scanning autocorrelator employing a 1-mm-thick KDP crystal; for spectral measurements we used an OMA III multichannel analyzer (EG&G). For energy measurements we used a pyroelectric ratiometer (RJ-7620, Laser Precision Corporation).

Fig. 1. (a) Numerically simulated pulse shapes: the SH pulse at compressor output (dashed curve) and the TOPG signal wave at 1040 nm (solid curve). ( b) Autocorrelation measurements of the SH pulse and TOPG signal pulse. Dashed and solid curves show Gaussian fits for data points; numbers indicate pulse duration at FWHM.  1995 Optical Society of America

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Before the experiments a numerical simulation of all the processes involved was carried out with software for modeling the three-wave interaction. Assuming 1.3-ps input pulses and an intensity optimized for minimal magnitude of the prepulse and the afterpulse (12 GWycm2 ), the calculation yielded a 200-fs-wide central peak and two satellite pulses shifted by , 500 fs from the main pulse [Fig. 1(a)]. The satellite pulses contained , 11% of the total energy. Experimental data were consistent with the simulation. The presence of two shoulders on each side of the autocorrelation trace indicated that the SH pulse consisted of three subpulses [Fig. 1(b)]. The decomposition of the SH autocorrelation trace into three Gaussian curves yielded 280-fs-wide central peak; the corresponding pulse width then was 200 fs. Energy conversion as measured in the whole beam was , 45% at the frequency-doubler input pulse energy of 2.5 mJ. It should be noted that the spatial prof ile of the laser beam at the SH compressor was somewhat f lat-topped because of saturation in the regenerative amplifier laser rod and diffraction on the rod aperture. Numerical simulation of the parametric amplif ication was performed in the plane-wave approximation with the group-velocity mismatch and pulse spreading effects taken into account. We assumed pump intensities of three subsequent TOPG stages to be 115, 50, and 25 GWycm2 for the seeder, preamplif ier, and power amplifier stages, respectively. In the 4-mmlong BBO crystal this provided corresponding gain factors of 107 , 104 , and 200. At the input of the seeder, instead of quantum noise, we assumed a 1.5-ps-long Gaussian-shaped pulse. Energy conversion was kept at ,10% in the first two passes and at ,15% in the third one. The optimal delay between the signal and pump pulses for the highest energy conversion was found to be 270 fs. The prof ile of the pulse exiting the TOPG is depicted in Fig. 1(a). It is interesting that only a weak prepulse is generated on the leading wing. This is because the idler pulse escapes the main pump pulse and interacts with the pump prepulse, thereby producing radiation at the signal wavelength caused by the difference-frequency generation. The experimentally measured autocorrelation trace followed closely the Gaussian shape over 3 orders of magnitude, which was the dynamic range of the autocorrelator. Suppression of the satellite pulses also changed the shape of the spectrum. It became rather featureless (Fig. 2); the time–bandwidth product was ,0.6 within a signal wavelength range of 635–650 nm and somewhat less in the rest of the signal tuning range. The total energy conversion, defined as (Es 1 Ei dyEp , was ,10% for the TOPG as the whole and ,15% for the last amplif ier stage [Fig. 3(a)]. This is less than the efficiency of a similar device pumped by uncompressed pulses [,40% (Ref. 7)]. We attribute this difference mainly to the fact that a signif icant amount of the pump pulse energy is contained in the satellites. On the other hand, the power amplifier stage in the present experiment was pumped by lower intensity to suppress the satellites. The wavelength range in which pulse duration measurements were possible was limited to 1200 nm from

the long-wavelength side by the design of the autocorrelator. Within this range, we did not observe strong deviation from the value of 200 fs [Fig. 3(b)]. With the pump wavelength of 527 nm, a rather special situation with the group-velocity mismatch takes place in the BBO type II phase-matching crystal. In brief, the 200-fs signal pulse has a splitting length of 8–40 mm, but that of the idler is only 1.3 –2 mm, depending on the wavelength. The splitting (walkoff) length is def ined as the propagation distance at which the initially overlapped pump and signal (idler) pulses separate from each other, in the absence of gain, as a consequence of the group-velocity mismatch. Therefore one may expect different behavior of the signal and idler pulses with respect to the dependence of pulse duration on the crystal length. We tested this point by modeling and measuring pulse durations in

Fig. 2.

Spectra of pump and signal pulses.

Fig. 3. (a) TOPG with 4-mm BBO crystal energy output characteristics; ( b) TOPG with 4-mm BBO crystal pulse duration versus wavelength.

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cosecond Nd:glass laser. For a 1.1-mJ, 200-fs pump, the single pulse tunable from 635 to 3000 nm has the energy of tens of microjoules and a duration in the range of 170–230 fs. Dramatic suppression of the pump pulse wings during the parametric process was observed. In contrast to numerous reports on the use of the parametric process for pulse shortening, owing either to high nonlinearity of the process or to special group-velocity conditions8 – 11 we can operate the TOPG close to saturation, thereby keeping energy conversion and f luctuations at a reasonable level. We believe that the high contrast and spatial quality (,1.1 times the diffraction limit) and the subgigawatt peak power of the nearly transform-limited pulses make the TOPG combined with the SH compressor an attractive tool for pump-and-probe experiments as well as for studies in nonlinear optics. In some cases, the temporal resolution of a system based on such a source may approach that attainable with a regular Ti:sapphire femtosecond oscillator/regenerative amplif ier. This research was made possible in part by grant LA4000 from the International Science Foundation. Fig. 4. (a) Numerically simulated pulse shapes at the TOPG output. The solid curve illustrates the idler pulse in the case of the 4-mm crystal; the dashed curve shows the idler pulse in the case of the 8-mm crystal. Numbers refer to FWHM of fitted Gaussian-shaped pulses. ( b) Experimental data from the TOPG with the 8-mm crystal output. Autocorrelation traces of the signal and idler pulses, measured close to degeneracy (ls ­ 1040 nm, li ­ 1070 nm).

crystals of different lengths (4 and 8 mm). We found that the signal pulse duration changes a little when the crystal length is doubled, whereas the idler pulse broadens remarkably. Figure 4(a) shows calculated pulse shapes of the idler pulse close to degeneracy for 4- and 8-mm crystal lengths. Experimentally, in the 8-mm-long crystal we have observed broadening of the idler pulse by a factor of ,2 compared with the signal pulse [Fig. 4(b)]. Thus if only the signal branch is to be used as the TOPG output, rather long crystals, which require lower pump intensities, may be suggested. In conclusion, we have demonstrated femtosecond BBO TOPG pumped by the self-compressed SH of a pi-

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